Wide-bandgap materials, notably GaN, offer high breakdown field and carrier mobility, as well as chemical and thermal stability. Consequently, devices employing this material have applications in high power switching circuits, as well as high frequency amplifiers. A significant performance- and reliability-limiting factor is the high defect densities that result from heteroepitaxy on SiC, Si, or sapphire substrates due to the lack of readily-available bulk GaN. Furthermore, the lack of a controllable p-type doping process significantly limits the circuit applications, since complimentary transistors and diodes cannot be built. Therefore, there is a need for a method to reduce the defect density and/or improve dopant activation
A reliable ion implantation process would be beneficial for GaN device manufacturing, as it would enable selective area n- and p-type doping. See K. T. Liu et al., “Crystal Polarity Effects on Magnesium Implantation into GaN Layer,” Japanese Journal of Applied Physics, 2010, 49(7). To realize such a process, post-implantation damage removal and electrical activation by annealing are critical steps to achieving a high quality GaN crystal with high mobility and high carrier concentration.
To date, a common problem in GaN device technology has been the low yield of activation of uncompensated Mg acceptors. Achieving a sufficient p+ hole concentration requires an adequate high temperature annealing of defects in the GaN after implantation.
Annealing structural defects in GaN is a challenging task because of the thermodynamic instability of GaN at high temperatures. Equilibrium nitrogen pressure above GaN increases quickly with temperature, and exceeds 1 bar at 850° C. (only ⅓ of the melting point). At the same time, it is known that estimated annealing temperature required for sufficient diffusion is about ⅔ of the melting point of the material, e.g., about 1400-1500° C. in the case of GaN. See S. J. Pearton et al., “Ion Implantation in Group III Nitrides,” Comprehensive Semiconductor Science and Technology, 2011, Elsevier: Amsterdam, pp. 25-43.
Annealing GaN at these temperatures can remove defects induced by the implantation, and move implanted Mg impurities to the proper lattice sites to substitute for Ga, making them electrically active. See H. Alves et al., “Mg in GaN: the structure of the acceptor and the electrical activity,” 2003, WILEY-VCH Verlag. p. 1770-1782. However, to prevent GaN dissociation at 1500° C., a nitrogen pressure of at least 15 kbar is needed. See S. Porowski et al., “Annealing of GaN under high pressure of nitrogen,” Journal of Physics: Condensed Matter, 2002. 14(44): p. 11097-11110, showing that annealing of GaN is possible without surface degradation at temperatures up to 1500° C. under N2 pressure of 16 kbar. According to the photoluminescence (PL) spectra, magnesium was optically activated after the implantation when annealed above 1300° C., and optical activity increased with increasing annealing temperature up to 1500° C. Id.
Rapid Thermal Annealing (RTA) is a commonly used technique for electrical activation of dopants after implantation in many semiconductor materials. In the case of GaN, RTA permits annealing of GaN at temperatures above its thermal stability without apparent GaN decomposition. If GaN is annealed at high temperatures for a short time, only a small amount of nitrogen has time to leave the surface of GaN, while the material in bulk remains unaffected. To prevent nitrogen loss from the surface during RTA and to keep the GaN surface from degrading, one can use an annealing cap on the top of GaN. Different materials, such as SiO2, Si3N4, and AlN have been used as caps in studies of GaN high temperature annealing. See, e.g., Y. Irokawa et al., “Electrical characteristics of GaN implanted with Si+ at elevated temperatures,” Applied Physics Letters, 2005, 86(11): p. 112108; S. Matsunaga et al., “Silicon implantation in epitaxial GaN layers: Encapsulant annealing and electrical properties,” Journal of Applied Physics, 2004, 95(5): p. 2461-2466; and X. Cao et al., “Redistribution of implanted dopants in GaN,” Journal of Electronic Materials, 1999, 28(3): p. 261-265.
A combination of RTA and GaN capping with AlN has been shown to be an effective method for heating GaN up to 1500° C. for very short times without losing nitrogen from its surface. See G. S. Aluri et al., “Microwave annealing of Mg-implanted and in situ Be-doped GaN,” Journal of Applied Physics, 2010, 108(8): p. 083103. However, no electrical activation was demonstrated in Mg implanted samples even after 15 seconds of annealing at 1500° C. due to the high residual implant damage remaining in the material. Id.
The motivation for this invention came from the understanding that GaN annealing is a serious hurdle in the development of a large variety of GaN based devices Annealing at high N2 pressure allows the heating of GaN at high temperatures with the GaN being thermodynamically stable for a long time without GaN dissociation, but the use of such high gas pressures requires unique high pressure equipment and time-consuming procedure to load samples and apply high N2 pressure. This makes high-pressure annealing too expensive for industry and difficult to scale up to lager wafer sizes.
RTA allows annealing of GaN at temperatures above GaN stability and therefore is an attractive technique for GaN annealing in terms of achievable temperatures and costs. However, it has not been shown that RTA enables electrical activation of Mg after the implantation in GaN without co-doping with other ions. See K. T. Liu et al., “Magnesium/nitrogen and beryllium/nitrogen coimplantation into GaN,” Journal of Applied Physics, 2005, 98(7): p. 073702-5. One reason for these unsuccessful attempts of using RTA to activate the implanted Mg is the short duration of the GaN annealing at sufficiently high temperatures. A new technique is required to enable a long duration annealing at high temperatures above thermal stability of GaN. Mg-implanted electrical activation and demonstration of the p-type conductivity with use of a new annealing technique would be a strong evidence of the efficiency of this technique.